Digital optical focal length modulator

ABSTRACT

A DIGITAL FOCAL LENGTH MODULATOR ASSEMBLY WHERE A NUMBER OF ELECTRO-OPTICALLY CONTROLLED ALIGNED STAGES, DIGITALLY CONTROL THE FOCAL LENGTH OF THE ASSEMBLY. IN EACH STAGE OF THE ASSEMBLY AN ELECTRO-OPTICALLY CONTROLLED POLARIZER DIGITALLY ALTERS THE EFFECTIVE FOCAL LENGTH OF A PAIR OF BIREFRINGENT LENSES. THE EFFECTIVE CURVATURE OF THE LENSES PROGESSIVELY INCREASES ALONG THE ASSEMBLY.

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DIGITAL OPTICAL FOCAL LENGTH KODULATOR Filed April 7. 1969 fig.3 L-

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United states Patent once 3,565,510 DIGITAL UPTICAL FOCAL LENGTH'MODULATOR Uwe Schmidt, Pinneberg, Germany, assignor to U.S. PhilipsCorporation, New York, N.Y.,z a corporation of Delaware Filed Apr. 7,1969, Ser. No. 813,940

Claims priority, apllilitigti Germany, Apr. 6, 1968,

U.S. Cl. 350--175 A The invention relates to a digtal focal lengthmodulator having a plurality of stages which are arranged one behind theother and each comprise a controllable electro-optical polarizer and abirefringent lens.

Since a modulator is known, see e.g. U. I. Schmidt, The Problem of LightBeam Deflection at High Frequencies in Optical Processing of Infomation,pp. 98-103, Spartan Books, Baltimore, 1963. In the known modulator alight beam is caused to pass through a sequence of electro-opticalpolarizers alternating with birefringent lenses. By applying suitableelectric voltages to the polarizer, with suitable orientation of theoptic axes of the birefringent lenses the angnlar aperture of the lightbeam can be changed in digitalhr steps. In this method, the variation ofthe focal length as such is solely determined by the geometry of the`birefringent lenses and by the values of the refractiveindices of thebirefringent material and of the refractive index of the surroundingmedium. The code of the' voltages to be applied to the polarizers isbinary.

The change of the focal lengths takes place in discrete steps. If thefocal lengths of ,the birefringent lenses of the individual modulatorstages are chosen so as to increase from stage to stage by a factor of 2and if the thinlens laws apply to these lenses, each available focus isspaced from the adjacent foci by the same distances.

The condition that the birefringent elements are thin lenses is notalways satisfied, because the requirement that the focal lengths form ageometrical progression may readily lead to very small focal lengthsi.e. to large aperture ratios. In this case, there is no longer anycertainty that the said equality of spacings between the-foci isactually obtained.

It is an object of the invention to reduce the differences of thespacings between the foci. For this purpose,

the invention is characterized in that in each stage at least twobirefringent lenses are arranged one behind the other,

the optic axes of the two lenses being normal to one another and to thenormal principal planes of the two lenses whilst the facing surfaceshave opposite curvatures.

Featuresand advantages of the invention will appear from the followingdescription of embodiments thereof, given by way of example only, withreference to the accompanying drawings, in' which:

FIGS. 1, 2 and 3 show schematically embodiments of a modulator accordingto the invention, and

FIG. 4 shows schematically a lens for use in an embodiment.

5 Claim' 2 'Y -l. '1 ,i Referring te F1G.',1taere" ashowa schenriieauythe structure of a digital focallength modulator.-The digital focallength modulator comprises af sequence of N polarizers PL. PNalternating with, N birefringent ,lens

combinations-L! L -which each comprise: a lconcave lens Lk anda convexlens `L,. In the embodiment described the value. of the eectivecurvatures lc! N and k"1.. kN,. of. the individual lenses increases by afactor of two in the direction of propagation of a light beam S passingthrough-the focal length modulator. The term effective curvature4 is tobe understood to mean the reciprocal of the sum of the radii ofcurvature of the active lens surfaces, the sign of the curvature. beingused in the usual manner. As has beenmentioned hereinbefore, each stageof the modulator includes a concave lens Ik andaconveielens-lv madeofauniaxialbirefringent material. The optic axes a and a' of the twolenses," are normal to one another and to the normal, vto the principalplane. In addition, in modulators which exhibit the transverseelectro-optic etfect the.two optic axes make an .angle of 45 with thedirection of the electric field' to be applied to the electric-optiemodulator. This orientation of the electro-optic modulator ensures thata light beam whichl initially was plane polarized parallel to one opticaxis and hence at right angles to the other optic axis, on

ing through the focal length modulator 'always remains polarized atright angles to` and parallelto, respectively, the optic axes,neglecting aberrations.

Since each stage of the focal length modulator includes two birefringentlenses, the curvature of the active refracting surfaces is reduced incomparison 4to a single lens having the same refractive power as the twolenses together. In addition, the use of ,a concave lens and a convexlens in each stage provides a thinner lens structure than the use of twolenses having surfaces in which the signs of the curvature are equal.The two aforementioned factors result in that the laws of thin lensesfrequently will apply in cases in which they would not suciently applyif single birefringent lenses were used.

The lens construction described may be varied in several manners withoutdeparting from the scope of the invention. The system shownschematically in FIG. 2 by using plane outer surfaces p and p' of thebirefringent lens combination L1 LN ensures that thel locations of thefoci are independent, in a degree suicient for most practical uses, ofthe uctuations of the refractive index of the medium surrounding thelenses due to temperature variations. It is assumed that within eachlens combination two refractive surfaces arranged as close to oneanother as possible and facing one another show complementary shapes,i.e. have oppositely equal curvatures.

A further property of the arrangement shown in FIG. 2 is that the rangeof modulation of the angular aperture ofthe beam S entering the systemis always equal in the directions towards smaller and greater values,neglecting slight aberrations. -In general, it will not be necessary forV all the stages of the focal length modulator to be combinations whicheach comprise more than one birefringent lens. In the stages to be usedfor effecting small variations of the angular aperture of the light beamsingle lenses will be sufficient. as is shown schematically in FIG. 3,only the last lens combination I. consisting of a` concave lens Ik and aconvex lens 1,.

If a combination comprising two birefringent lenses should not besufficient to reduce the said aberrations, according to the invention afurther improvement is obtainable by the use of a greater number ofbirefringent elements per stage. FIG. 4 shows an example ofa-birefringent lens which consists of three lens elements I1, I3 and I3.

, fastened Feb. z3, ,1911* l ,f .u r In lens combinations according tothe invention it is not absolutely' necessary 'to use separate elementsthe two perature i variations. Whichv combinations of curvatures ofrefracting surfaces are -most suitable in aparticular case maybeldecided by means ofthe known methods ofO correcting optic'al'systemsandaccordingfto th'e laws of double` refraction, as has been described,for example by I-Flggez' "Leitfaden der geometrischen Optik und destOptikrechnens, Vandenhoeck und Ruprecht, Gttingen, 1956, and byP.'Chmel:vStrahlenverlauf in einem Systemaus doppelbrechenden Linsen,Optik, 26, 254-263 (.1968). 2i

' What is claimed isz' 1. A digital focal length modulator assemblycomprising la plurality of optically aligned modulator stages, eachofthe stages comprising a' controllable-electroioptical polarizer and abirefrigsgen't lens,l lthe lenses of each' successive stage having aprogressively increasing effective curvature, the lens having thelargest effective curvatu're comprising at least two confrontingbirefringent lenses having mutually perpendicular optic axes arrangednormal to the principal planes of the lenses, the facing surfaces of theconfronting birefringent lenses having opposite curvatures.'`

. in each stage" having opposite curvatures.

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2. Apparatus as claimed in claim 1 wherein the nonfacing major surfacesof the confronting lenses are planar.

3. Apparatus as claimed in claim 2, wherein the lenses in substantiallyall of the stages comprise at least two confronting birefringent llenseshaving mutually perpendicular optic axes arranged normal to theprincipal planes ofmthe lenses, the facing surfaces` of the confrontinglenses 4. Apparatus as claimed in claim 2, wherein the lenses insubstantially-all offth'e stagesc'omprise at least two `confrontingbir'efringent lenses having mutually perpens'soi-SO, 204, 214

dicular optic axes arranged normal to the principal planes of thelenses, the facing surfaces of the confrontinglenses in each stagehaving opposite curvatures.

5. Apparatus as claimedin claim 1, wherein the lens comprises a firstlens of one ty-pe of -cur'vature interposed between a pair of additionallenses of an opposite curvature.

` References Cited ,UNITEDLYSTATps PATEN'Ts 410,624 lil/1968 Schmidt350-175(DRL) 3,432,238; .3/1969 Girard- 35o-179x JOHN K. CRBIN, rrimarynimmer U.s. C1. x,R.

